Single crystal gallium nitride is a technologically important material finding increasing use in high frequency RF devices, and Light Emitting Diodes (LEDs). In the absence of methods to form single crystals of this and similar materials from melt, they are invariably grown by hetero-epitaxy by metal-organic chemical vapor deposition, M-O-CVD, or by atomic layer deposition, ALD, on single crystal substrates of sapphire (Al2O3), or silicon carbide (SiC), because of their refractory nature, purity, inertness, and reasonably close lattice structure match to gallium nitride. Both sapphire and silicon carbide are in themselves extremely hard to grow as single crystals, the larger the diameter, the harder to make them. Until recently, nearly 90% of gallium nitride crystals were grown on 2-inch diameter. Only in 2009 this percentage dropped below 50%, and now most new LED fabricators are using 4″ substrates, and some even venturing into 6″ diameter sapphire wafers. Growing GaN on single crystal silicon carbide is somewhat easier because of closer lattice matching, but silicon carbide wafers are stuck at 2″ diameter. GaN growth, on the small diameter sapphire wafers entails an enormous loss of productivity. This is a great impediment to them affordable for replacing the incandescent lighting. It is for this reason that there has been a continuing effort to use silicon wafers as substrates for GaN Epitaxy.
If silicon wafers can be used easily for growing gallium nitride, the advantages of larger wafer sizes, wide availability, atomically smooth growth surfaces, would quickly lead to their wide adoption. Why is this not the case? Growing GaN epitaxially on silicon (111) would face both a larger lattice mismatch (17%), and a larger thermal expansion mismatch (about 50%). Researchers have been able to bridge the lattice mismatch the same way as is done in cases of sapphire and silicon carbide, here using buffer layers of AlGaN to grow low defect GaN films on silicon. This greatly reduces the lattice strain in GaN layer and, as a result, reduces the dislocation density to reasonable levels. However, the sign and magnitude of thermal contraction mismatch between GaN and silicon, are such to give rise to extensive cracking of the latter upon cooling. In practical terms, this limits the thickness and size of useful devices, and the yield of such devices.
Some ingenious methods for growing GaN on silicon have been developed to enable the use of silicon substrates for GaN growth. Almost all these methods are based on modifying the growth surface with a) use of multiple or varied buffer layers, b) limiting the size of crystals growing and of crack prorogation by scoring the silicon wafer surface, c) limiting growth surface with in-situ silicon nitride masking, and allowing for lateral growth over the masked areas to fill the surface, and d) to change the growth morphology to nano rods. Even with these difficulties, after years of development, limited commercial production of GaN on silicon substrates has just begun.
The one case where a silicon wafer was modified on the non-growth side missed the mark. They attached very thin silicon 111 wafer, or very thin single crystal silicon carbide wafer, to polycrystalline silicon carbide wafers, apparently to reduce cost of the growth wafers. They missed the mark in the sense, that the support wafer bonded to the growth wafer, either had the same or similar coefficient of thermal expansion to silicon, in one case, and silicon carbide, in another, to make any difference in the cracking behavior. Even then, the researchers reported growing good quality GaN epitaxial layers on 2″ substrates.